Liquid crystal displays (LCDs) are used in portable computers and other electronic devices to display information. LCDs modulate light to create images using selectively transmissive and opaque portions of the display, the selection being controlled by passing electric current through the liquid crystal material. LCDs are available in transmissive, reflective and transflective versions. A transmissive LCD is illuminated by an artificial backlight positioned behind the LCD glass to provide the contrast between the light transmissive and opaque portions of the display. The transmissive LCD can be adequately viewed only when the backlight is illuminated. When the backlight is illuminated, this type of LCD is very readable in low to medium light, with a typical contrast ratio of 18:1 for the very best LCDs. Under strong lighting conditions, however, the contrast ratio is significantly lower, making the LCD more difficult to read. Another drawback to transmissive LCDs is that the backlight consumes a significant amount of power making them somewhat undesirable for use with portable devices, in which power conservation is a concern.
A reflective LCD has a rear reflective surface and is illuminated from the front of the display, typically by ambient light similar to LCD watches, to provide the contrast necessary for readability. Reflective LCDs provide a fairly high contrast in medium to strong light, but are difficult, if not impossible to read in low or nonexistent light. On the positive side, however, because a backlight is not used, reflective LCDs consume considerably less power than their transmissive counterparts.
A transflective LCD is a combination of the other two types of LCDs and operates in both transmissive and reflective modes. A one way reflective layer behind the display reflects ambient light striking the front of the display, and also permits light to be transmitted through the display from a backlight. In bright or average ambient light conditions, the LCD is adequately illuminated from the front by light reflecting from the reflective side of the layer. In low ambient light conditions, the backlight may be activated to illuminate the display through the transmissive side of the layer. Although transflective displays combine many of the benefits of the other two types of displays, a major drawback to such displays is that, due to the reflective layer, a very strong backlight is necessary to sufficiently illuminate the screen in low light conditions, such that a transflective LCD will consume more power with the backlight on than will a normal transmissive LCD.
Transmissive or transflective LCDs are popular in personal computer systems because backlighting as a source of illumination provides the necessary contrast ratios and brightness for viewing the display. However, as previously indicated, relatively large amounts of light are required to light the display due to the extremely low percentage of light transmission through the display. For example, monochrome LCDs are only approximately ten percent transmissive. Full color, active matrix LCDs have absorptive dies and addressing structures that only allow light transmission in the range of five percent or less. Transflective LCDs further limit light transmission because the one way reflective layer is not highly transmissive. While backlit LCDs have several advantages in that sufficient light is always available, they are very inefficient in their use of power to provide an acceptable display.
An additional problem associated with the use of transflective LCDs, as opposed to reflective LCDs, is their relatively poor ability to reflect ambient light. A reflective LCD, for example, is capable of being used in lower light conditions than is a transflective LCD. Thus, as ambient light falls below average, a transflective LCD must turn on its backlight, and thus drain precious battery power, before a reflective LCD would become unreadable.
LCD power consumption is a major concern in portable devices which run on batteries. Various power management techniques have been devised which operate to disable the LCD backlight and thus "blank" the display during periods of non use, with significant power savings. Unfortunately, such techniques tend to blank the screen at times inconvenient to the user, and further do not correct the fundamental power inefficiencies of backlighting systems.
Reflective LCDs, which are adequately illuminated in high ambient light conditions, have also been equipped with artificial front lighting systems, for use in low light conditions. Front lit reflective systems consume less power than comparable backlit transflective systems because there are no transmission losses effected by the one way reflective layer. A front lit reflective LCD therefore is an attractive alternative to a backlit system because of this reduced power requirement in combination with the ability to view the display in high ambient light without artificial illumination.
However, there are several problems associated with conventional front lighting systems for reflective LCDs. Many designs do not uniformly illuminate the display because the light source is located at one or more sides of the display to avoid obstructing its view.
The glancing angle, or angle of incidence, of light impinging on the surface of the display decreases with distance from the light source, with less light entering the display as the angle decreases. As used herein, "glancing angle" and "angle of incidence" are used interchangeably to refer to the angle at which a ray of light strikes the surface of the LCD. At sufficiently low glancing angles, light is completely reflected and does not illuminate the display. One design developed for relatively small LCDs provides a uniformly lit display by mounting the light source in a box like structure placed over the display. The structure has reflective side walls that surround the display to define a viewing window and to support the light source elevated above the display. While efficient to illuminate a compact LCD, the design is cumbersome and not readily adaptable to larger LCDs where a relatively flat LCD panel is preferred. Moreover, the walls of the structure tend to partially obstruct the view of the display to an extent unacceptable for most applications.
What is needed is a front lighting system for a reflective LCD that uniformly illuminates the display without obstructing its view and which provides for an unobtrusive, relatively flat LCD panel configuration. This would be especially useful for portable electronic devices with relatively large LCDs where size and power consumption efficiency are important considerations.